Storage systems with a variety of performance levels have been developed. Volumes that constitute storage systems come in a variety of performance levels. Typically, volumes with high performance are expensive and have a low storage capacity, whereas volumes with low performance are inexpensive and have a high storage capacity.
There is known a data management method called an HSM (Hierarchical Storage Management) function that optimally arranges files through the use of a plurality of such volumes with different properties, with a view to reducing the cost of storage systems. With the HSM function, files which are frequently used are migrated to a “high-speed, expensive” volume, whereas files which are less frequently used are migrated to a “low-speed, inexpensive” volume (drive). Further, the HSM function makes such file migration transparent to clients. Controlling the volumes for storing files in this manner with the HSM function allows a reduction in the cost of the storage.
As a migration destination of a file, it is also possible to use another file server. Migrating a file to a file server with a “lower-speed, less expensive” volume allows construction of a more layered hierarchical structure and a further reduction in the cost. For example, Patent Literature 1 discloses a method of using another file server as a file migration destination. According to Patent Literature 1, after a file is migrated from a given file server to another file server, it is determined, upon request for access to the file, if the file is a stub file. If the file is determined to be a stub file, the request is transferred to the migration-destination file server to perform the processing.
In recent years, so-called cloud computing, with which a huge computing environment or storage over a network is used as a pay-as-you-go service via the Internet, or cloud storage (hereinafter also simply referred to as a “cloud”) has been spreading. Using such cloud storage as a migration destination of a file is also considered. Various types of storage management such as capacity management or failure management are carried out by vendors that provide cloud storage. Thus, it is expected that management cost be reduced by migrating files to such cloud storage.